I wouldn't go crazy buying an expensive engine for this perticular aeroplane. It's, well...OK if you know what I mean.
A .46FX is massive power and yes it's heavy. Flew mine with a cheapo .46LA... plenty of up-and-go..and light.
Yet it still needed a heep of lead in the tail to get the CG where it needs to be.
The 40LA you have will be more than enough.